Question #25124

Notice that the problem is asking for

total volume of gas, measured at #29^@"C"# and #"769 torr"#

which can only mean that your goal here to find the volume of carbon dioxide, #"CO"_2#, produced by the reaction.

So why carbon dioxide and not sodium carbonate, #"Na"_2"CO"_3# ? Well, because carbon dioxide is the only gas produced by the reaction. The sodium carbonate is solid, as you can see from the #(s)# symbol, and the water is liquid, as you can see from the #(l)# symbol.

Now, the problem provides you with mass of sodium bicarbonate, #"NaHCO"_3#, and mentions that all the reactant is converted to sodium carbonate.

This is just a tricky way of saying that all the mass of sodium bicarbonate reacted to produce sodium carbonate. However, sodium carbonate is not the only product of the reaction.

Water and carbon dioxide will also be produced when the sample of sodium bicarbonate undergoes decomposition.

As you can see in the solution, you can ignore the sodium carbonate completely because it is not important here.

Once you convert the mass of sodium bicarbonate to moles, you can use the #2:1# mole ratio that exists between the reactant and carbon dioxide to find the moles of carbon dioxide produced by the reaction.

Once you know that, the ideal gas law equation helps you find the needed volume.